1. Field of the Invention
This invention is in the field of drying and pulverizing coal to supply coal dust serving as a fuel in the manufacture of cement, the cement making system including a pre-heater, a calcinator, a rotary tubular kiln, a cooler, as well as a tertiary air conduit supplying the calcinator with heated air from the cooler.
2. Description of the Prior Art
Systems for cement manufacture frequently employ the so-called direct-fired coal mills for firing the rotary tubular kilns in which the raw material for cement is sintered. In these direct-fired coal mills, raw coal is simultaneously ground and dried. The blower, which blows hot air through the mill from the cooler for drying and screening the coal simultaneously, serves as the primary air blower. The pulverized coal is blown directly from the mill into the kiln. Raw coal is distributed to the mill by means of a hopper and a metering device which is driven by a controllable motor operating in accordance with the required conditions of operation. The discharge of coal dust into the rotary tubular kiln does not depend solely on the speed of the metering apparatus in front of the mill, but also depends on the amount of air which is introduced into the kiln by means of the mill blower. By means of controlling the speed of the mill blower, the coal dust discharge and the flame position in the rotary tubular kiln can be affected. (See, for example, the periodical "Zement-Kalk-Gips", No. 11, 1956, Page 491). In this publication it is stated that automatic operation in rotary tubular kilns with direct-fired coal mills has not been yet capable of realization.
The difficulty results because the discharge of coal dust and the injection of primary air into the rotary kiln are functionally coupled to one another and are therefore closely interconnected. An increase of the amount of primary air, for example, by increasing the speed of the mill blower results in more coal dust being delivered to the kiln. In so doing, however, the grain structure changes. The coal dust no longer has the same degree of fineness as previously because the sifting means between the mill and the kiln allows coarser ground matter to pass through because of the greater amount of air. Accordingly, the formation and the position of the flame cone in the kiln is significantly changed. It is possible to achieve and maintain a more or less stable kiln operation only by means of very careful servicing by trained personnel. With burning systems at the present state of development, particularly with calciners and secondary firing, the difficulty in coupling a coal grinding system to a burning system increases significantly due to the necessity for correlating the amounts of powdery coal and gas.